I am thinking now that the problem is not with the 504, but with the head unit of the stereo. When I turn down the volume of the 504, the hissing is constant. If I turn down the 504 volume to 1/2 or...

I use a 504 to play music through the aux input of a car stereo. Sometimes, there is a good deal of hiss such that I have to turn down the aux input gain on the stereo to minimum (-8dB). Other times...